The present invention relates to a facsimile system by which highly efficient transmission can be accomplished through reduction of redundancy, during transmission of picture signals, where binary information containing redundancy is handled.
The conventional highly efficient facsimile transmission systems are the following two:
(a) A code conversion redundancy reduction system for the reduction of redundancy inherent to the document/picture, which are the information sources; and
(b) A highly efficient modulation/demodulation method for the purpose of minimizing the transmission channel redundancy which does not effectively utilize the transmission frequency bandwidth.
As a modulation/demodulation method for voice channel telephone circuit used for the transmission of document/picture, AM-DSB (Amplitude Modulation-Double Side Band) or FM (Frequency Modulation) is designated for the Group 1 model (6 minute machine) of CCITT; AM-PM-VSB (Amplitude Modulation-Phase Modulation-Vestigial Side Band) is designated for the Group 2 model (3 minute machine) of CCITT. For the Group 3 model of CCITT (1 minute machine), V26 bis (2400 bits/sec) or V27 ter (4800 bits/sec) are recommended by CCITT. The recommendations by CCITT are published by ITU (International Telecommunication Union) located in Geneva in Switzerland, and ITU is one of the subsidiary organizations of the United Nations. Thus, the use of PM (Phase Modulation) is being encouraged. On the other hand, with respect to the redundancy reduction method, various propositions are being made for the attainment of high speed (about one minute) transmission of A4 size (appoximate 21 cm.times.29.5 cm).
For any model higher than the Group 3 model of CCITT (1 minute machine), one of these technologies is likely to be adopted. These provisions focus on the run length of the same picture element within a single main scanning line, and a method of having this run length match with the Wyle's code or having the run length match with the modified Huffman code, or any other one dimensional run length encoding methods, has been proposed. Further, various inventions relating to two dimensional encoding method have been announced.
Among the prior arts mentioned above, the AM-PM-VSB (Amplitude Modulation--Phase Modulation--Vestigial Side Band Modulation) modulation system and the skipping white space (SWS) redundancy reduction system, which are closely related to the present invention of highly efficient facsimile transmission system, are briefly explained below.
The AM-PM modulation system is shown in "CCITT" period 1977-1980 study group XIV-contribution No. 2, by Nippon Telephone and Telegraph Public Corporation), and is summarized below.
FIG. 1 is a schematic block diagram of a facsimile transmitter incorporating multilevel AM-PM modulation system which is a prior art. FIG. 2 is waveforms prepared to explain this modulation method. In FIG. 1, the picture element information of a whole line has been scanned. Black picture element having "0" level, and white picture element having "1" level are supposed to have been stored into the shift register 1 through the input line 100. The serial binary picture signals 102 read out by the timing signal 101 are applied to the polarity inversion circuit 3 and to the flipflop 2. The flipflop 2, at the changing point from the "1" level of the input signal 102 to the "0" level, inverts the status of the output 103 from L to H or from H to L (see FIG. 2). When the signal 103 is L level, the polarity inversion circuit 3 provides the output 104 which is the same as the input 102. When the signal 103 is H level, it provides the polarity inverted output of the input 102 to the output line 104. Therefore, as can be observed in the waveform diagram of FIG. 2, the signal 104 becomes ternary waveform comprising +1 level, 0 level, and -1 level. Further, the signal 104 receives a level shift of +1 level at the addition circuit 5 and gains the ternary output 105 comprising +2 level, +1 level, and 0 level.
On the other hand, the aforementioned signal 103 is also applied to the input of the flipflop 4. Therefore, at the changing point from H to L of the input signal 103, the flipflop 4 causes the status of output line 106 to invert from L to H or from H to L. By the input signals 105 and 106 thus created, the polarity inversion circuit 6 behaves in the same way as the inversion circuit 3 and provides the quinary baseband signal comprising +2 level, +1 level, 0 level, -1 and -2 level as illustrated in FIG. 2. At the amplitude modulation circuit 8, the output 108 of the carrier wave generation circuit 7 undergoes amplitude modulation by the aforementioned baseband signal 107, and provides quinary AM-PM modulation signal 109A as illustrated in FIG. 2. The AM-PM-VSB modulation wave for general practical use can be obtained by passing the quinary AM-PM modulation signal 109A through the VSB circuit.
The above describes the quinary AM-PM-VSB modulation operation. If the circuit 9 surrounded by the dotted line in FIG. 1 is excluded, the amplitude modulation circuit input is given to the signal 104 instead of the signal 107. In this case, the ternary AM-PM modulation signal 109B illustrated in FIG. 2 can be obtained for the output of the amplitude modulation circuit 8. Further, by passing through the VSB circuit, the ternary AM-PM-VSB modulated waves can be transmitted to the channel. As mentioned above, through use of the multilevel AM-PM-VSB modulation system, a highly efficient document/picture transmission, effectively utilizing a given frequency bandwidth, becomes possible.
However, it is evident that the above described multilevel AM-PM-VSB modulation system has yet to be improved in that the redundancy areas and the significant information areas in the picture are handled equally.
Following is a brief description of another prior art, skipping white space system (SWS) with reference made to FIG. 3 and FIG. 4. Concerning a prior SWS system, there are many documents, some of which are U.S. Pat. No. 4,044,382, and U.S. Pat. No. 4,047,228. FIG. 3 illustrates a schematic block diagram of the facsimile transmitter employing SWS system. FIG. 4 illustrates waveforms which explain the conventional SWS system. FIG. 4 provides an example where the number of constituent picture elements in one line counts 1728 which are divided into 108 blocks of 16 picture elements each. It is supposed that one scanning line of picture information has already been scanned. The black picture element "0" is made to match with the white picture element "1" and has been stored in the picture signal memory 20 through the input line 120. At the same time, with respect to the blocks, each of which has 16 picture elements there blocks without black picture elements are designated as flag "0". These flags 1 or 0 are given sequentially by the identification circuit 21. The results are supposed to be stored into the flag memory 22. The control circuit 23 reads out the flag for the first block from the flag memory 22 and, by finding that this flag is "1", identifies that this block has no block cell. In this case the control circuit 23 provides only flag "1" to the input 123 of the modulation circuit 24, but does not provide the content of the block. The same is repeated with respect to the second block. With respect to the third block (#3), the flag being "0", it is identified to be with black. In this case the flag "0" of the third block and the signal "1110000011100111" indicating the status of the 16 picture elements comprising the block (See FIG. 4) shall be an input of the modulation circuit 24 as a serial signal. The same processes continue until the 108 blocks have been transmitted and processing of 1 scanning line has been completed.
In this manner, in the conventional SWS system, the flag "1" that matches with the block without black cell has a role of block skipping flag. This substantially reduces transmission time. But, on the other hand, compared with the case where redundancy reduction were not done, the deterioration of the quality of reproduced picture would have been aggravated arising from transmission errors of the flags.
Further, in such a combination of AM-PM-VSB modulation system and the SWS redundancy reduction system, bit by bit performance is not necessarily satisfactory because of possible transmission errors, since characteristics of AM-PM-VSB modulation system lies in simplicity of its arrangement. In particular, it should be noted that the AM-PM-VSB modulation system is originally an analog modulation system and is not suitable to transmit a digital signal like a skipping flag. Therefore, a prior AM-PM-VSB system can not transmit a flag information (which is a digital signal) in SWS system, since a flag information is SWS system plays a major role and a single error of a flag information affects much to the deterioration of the picture quality.